This invention relates to infra-red transmitting materials, and in particular to the provision of surface coatings on such materials.
In many applications it is necessary that infra-red transmitting materials should be able to withstand somewhat rough treatment. Preferably such materials should be at least as wear resistant as visible light transmitting materials in this respect. For example, windscreen wiping to remove water is more necessary in the infra-red than the visible region of the spectrum as water is opaque over most of the infra-red region. Unfortunately many of the infra-red materials presently in use do not approach the hardness of glass. Those materials that are sufficiently hard, e.g. silicon, have relatively high refractive indices. Such a high refractive index results in spurious surface reflection which can be reduced only by the application of an antireflection film to the surface of the material. Such films, however are relatively soft and, as they are extremely thin, e.g. 1 micron, any wear soon results in the removal of the film. Even a diamond film of a micron thickness would not survive for any length of time.
To provide camouflage the eyes of moths have anti-reflecting surfaces; presumably a black cavity is less conspicuous than a reflecting surface. The anti-reflection surface evolved by the moth consists of myriads of pimples approximately half a wavelength of light deep spaced apart by less than the wavelength of light. The pimple arrangement, which does not have to be precisely regular, provides a quite effective anti-reflection coating.